Ring magnet and method of manufacturing the magnet

ABSTRACT

Radially oriented ring-shaped preliminarily molded pieces of a ring magnet are stacked in an axial direction to form a ring-shaped molded body, in which end faces of the ring-shaped preliminarily molded pieces are bonded together. This ring-shaped molded body is sintered and heat-treated to produce the ring magnet. Ring magnets which are less susceptible to deterioration of magnetic properties due to a disturbance of magnetic orientation near lamination boundary surfaces are produced. The invention provides a method of manufacturing such ring magnets with high efficiency.

TECHNICAL FIELD

The present invention relates generally to a ring magnet used in a rotorof a motor or the like. More particularly, the invention pertains to astructure for obtaining an axially elongate ring magnet and a method ofmanufacturing such a ring magnet.

BACKGROUND ART

Conventionally, a method of manufacturing a ring-shaped permanent magnetincludes the steps of filling powder of magnet molding material into acavity of a metal die unit and molding the powder of the magnet moldingmaterial by press working while magnetically orienting the magnetmolding powder by applying a magnetic field to the powder by means of apair of coils situated in a surrounding area of the metal die unit. Themetal die unit has a lower die section which includes a core, a lowerpunch and a die into which the core and the lower punch are inserted,creating a cavity in the lower die section. An upper punch positionedagainst the die and the lower punch pressurizes magnetic powder placedin the cavity.

If such a molding method is used for forming axially long cylindricalmagnets in forming these radially oriented anisotropic ring magnets in amagnetic field which are widely used in small motors, there arises aproblem that a sufficient degree of orienting magnetic field intensityis not obtained, resulting in a reduction in the degree of magneticorientation of the magnetic powder and an inability to achieve highmagnetic properties.

Generally, when a ring magnet is radially oriented magnetically, amagnetic flux passing through a core of a metal die unit for moldingmagnetic powder into a ring shape becomes equal to a magnetic fluxpassing within the inside perimeter of a die. Therefore, expressing theinside diameter of the ring magnet (the core diameter of the metal dieunit) as Di, the outside diameter of the ring magnet (the insidediameter of the die of the metal die unit) as Do, the height of the ringmagnet as H, the magnetic flux passing through the core of the metal dieunit as Bc, and the magnetic flux passing within the inside perimeter ofthe die as Bd, there is a relationship given by equation (1) below:2xp/4xDi ² xBc=pxDoxHxBd  (1)

A steel product, such as S45C, if used for the core of the metal dieunit, has a saturation flux density of approximately 1.5T. In this case,substituting Bc=1.5 in equation (1) above and assuming that a magneticfield necessary for magnetic orientation is equal to or larger than 1.0Twhich translates to Bd=1.0T, the height H of the ring magnet which canbe molded with magnetic orientation is given by equation (2) below:H=3Di ²/4Do  (2)

A problem which will arise when the ring magnet is formed in a magneticfield is a reduction in magnetic orientation performance that occurs ifthe axial length of the ring magnet exceeds the value of H of equation(2) above. Accordingly, conventional practice has been to produce ringmagnet pieces each having a short axial length which is equal to orsmaller than the value of H of equation (2) above and join them with abonding agent, for instance, to manufacture a ring magnet having anecessary axial length.

Also, as shown in Japanese Patent Application Publication No. Hei9-233776, for example, there is proposed a method of forming a magnethaving a necessary axial length by laminating molded magnet pieces in ametal die, each of the molded magnet pieces having a length that fallswithin a range in which these magnet pieces can be formed in a magneticfield.

Also, as shown in Japanese Patent Application Publication No. Hei10-55914, for example, there is proposed a method including the steps offorming preliminarily molded pieces in a magnetic field and joining aplurality of these preliminarily molded pieces into a single structureby pressurizing the same with a pressurizing force which is greater thana pressurizing force applied during preliminary molding.

While a conventional radially oriented anisotropic ring magnet ismanufactured by forming magnet pieces magnetically oriented in radialdirections and each having a short axial length in a particular shapeand bonding and stacking the magnet pieces with a bonding agent to makea ring magnet having a necessary axial length, this method is poor inproductivity. Additionally, this method has a problem that the accuracyof outside diameter worsens due to misalignment of central axes of theindividual magnet pieces when stacked, and unevenness of air gap occurswhen the magnet pieces are assembled with a stator, for instance,resulting in a deterioration in the precision in shape at assembly aswell as in magnetic properties.

Also, in the method in which a molded magnet unit having a necessarilylong axial length is formed by stacking molded magnet pieces each havinga short axial length in a metal die, magnetic powder is additionallyfilled on top of an already molded magnet piece and a newly moldedmagnet piece is formed thereon while applying a magnetic field.Therefore, there has been a problem that magnetic orientation in theproximity of a lamination boundary surface between the previously moldedmagnet piece and the newly molded magnet piece is likely to bedisturbed, resulting in a deterioration of magnetic properties.

Also, there has been a problem that magnetic orientation is apt to bedisturbed in the proximity of side surfaces of the molded magnet unit asthe side surfaces slide along the metal die when the already moldedmagnet unit is moved to a position below the metal die, resulting in adeterioration of magnetic properties.

Also, since an initially molded magnet piece is subjected to repeatedcycles of pressurization, there occurs a difference in the number ofpressurization cycles between the initially molded magnet piece and afinally molded magnet piece, resulting in a density differencetherebetween, consequently causing a problem that deformation occursduring sintering.

Also, in the method in which a plurality of preliminarily molded piecesare joined into a single structure by pressurizing the same with thepressurizing force which is greater than the pressurizing force appliedduring preliminary molding, there is a problem that a larger moldingfacility is required for repressurization and the preliminarily moldedpieces are susceptible to breakage at the time of repressurization.

This invention has been made to solve the aforementioned problems.Accordingly, it is an object of the invention to provide a ring magnetand a method of manufacturing the ring magnet which make it possible toachieve a reduction in deterioration of magnetic properties due to adisturbance of magnetic orientation in the proximity of each laminationboundary surface between laminated preliminarily molded pieces, as wellas high productivity and improved precision in shape.

DISCLOSURE OF THE INVENTION

A first ring magnet according to the present invention is a ring magnetproduced by stacking a plurality of radially oriented ring-shapedpreliminarily molded pieces in an axial direction to form a ring-shapedmolded body and sintering the aforementioned ring-shaped molded body.

According to this structure, it is possible to manufacture ring magnetswhich are less susceptible to deterioration of magnetic properties inboundary regions between the adjacent ring-shaped preliminarily moldedpieces with high productivity.

A second ring magnet according to the present invention is a ring magnetwhich is structured in such a way that a recess is formed in one of bothend faces in the axial direction of each of the aforementionedring-shaped preliminarily molded pieces while a protrusion is formed onthe other end face, and the aforementioned recess and the aforementionedprotrusion of the aforementioned ring-shaped preliminarily molded pieceswhich are adjacent to each other along the aforementioned axialdirection are fitted together.

According to this structure, it is possible to easily align central axesof the individual ring-shaped preliminarily molded pieces and therebyobtain a product that offers high precision in shape and preventsdisplacement of the individual ring-shaped preliminarily molded piecesduring transportation.

A third ring magnet according to the present invention is a ring magnetwhich is structured in such a way that the aforementioned recess andprotrusion formed in the aforementioned end faces are arc-shaped incross section.

According to this structure, it is possible to easily align central axesof the individual ring-shaped preliminarily molded pieces by fitting thearc-shaped recess and protrusion to each other and thereby obtain aproduct that offers high precision in shape and prevents displacement ofthe individual ring-shaped preliminarily molded pieces duringtransportation.

A fourth ring magnet according to the present invention is a ring magnetwhich is structured in such a way that the aforementioned recess andprotrusion formed in the aforementioned end faces are V-shaped in crosssection.

According to this structure, it is possible to easily align central axesof the individual ring-shaped preliminarily molded pieces by fitting theV-shaped recess and protrusion to each other and thereby obtain aproduct that offers high precision in shape and prevents displacement ofthe individual ring-shaped preliminarily molded pieces duringtransportation.

A fifth ring magnet according to the present invention is a ring magnetwhich is structured in such a way that the aforementioned recess andprotrusion are produced by making a stepped structure between an innerperipheral part and an outer peripheral part of each of theaforementioned end faces.

According to this structure, it is possible to easily align central axesof the individual ring-shaped preliminarily molded pieces by fittingstepped parts between the inner peripheral part and the outer peripheralpart of the end faces to each other and thereby obtain a product thatoffers high precision in shape and prevents displacement of theindividual ring-shaped preliminarily molded pieces duringtransportation. In addition, if the stepped parts are tapered, it ispossible to stack the preliminarily molded pieces with high precisionwithout damaging the preliminarily molded pieces in a stacking processthereof.

A sixth ring magnet according to the present invention is a ring magnetwhich is structured in such a way that the aforementioned recess andprotrusion are produced by forming a slant surface between an innerperipheral part and an outer peripheral part of each of theaforementioned end faces.

According to this structure, it is possible to easily align central axesof the individual ring-shaped preliminarily molded pieces by fitting theslant surfaces between the inner peripheral part and the outerperipheral part of the end faces to each other and thereby obtain aproduct that offers high precision in shape and prevents displacement ofthe individual ring-shaped preliminarily molded pieces duringtransportation.

A seventh ring magnet according to the present invention is a ringmagnet which is structured in such a way that the aforementioned recessand protrusion constitute a plurality of spherical parts formed atspecific intervals along a circumferential direction.

According to this structure, it is possible to easily align central axesof the individual ring-shaped preliminarily molded pieces by fitting theprotruding spherical parts into the recessed spherical parts and therebyobtain a product that offers high precision in shape and preventsdisplacement of the individual ring-shaped preliminarily molded piecesduring transportation.

An eighth ring magnet according to the present invention is a ringmagnet which is structured in such a way that the cross section of theaforementioned recess and protrusion is arc-shaped, V-shaped, U-shapedor trapezoidal.

According to this structure, it is possible to easily align central axesof the individual ring-shaped preliminarily molded pieces by fitting theprotruding arc-shaped, V-shaped, U-shaped or trapezoidal part into therecessed arc-shaped, V-shaped, U-shaped or trapezoidal part,respectively, and thereby obtain a product that offers high precision inshape and prevents displacement of the individual ring-shapedpreliminarily molded pieces during transportation.

A ninth ring magnet according to the present invention is a ring magnetwhich is structured in such a way that the aforementioned recess andprotrusion have a ringlike shape formed along the aforementioned endfaces.

According to this structure, it is possible to easily align central axesof the individual ring-shaped preliminarily molded pieces by fitting theprotruding ringlike part into the recessed ringlike part and therebyobtain a product that offers high precision in shape and preventsdisplacement of the individual ring-shaped preliminarily molded piecesduring transportation.

A tenth ring magnet according to the present invention is a ring magnetwhich is structured in such a way that the aforementioned recess andprotrusion constitute a plurality of parts formed to extend in radialdirections.

According to this structure, it is possible to easily align central axesof the individual ring-shaped preliminarily molded pieces by fitting aplurality of radially extending protruding parts into a plurality ofradially extending protruding recessed parts and thereby obtain aproduct that offers high precision in shape and prevents displacement ofthe individual ring-shaped preliminarily molded pieces duringtransportation.

An eleventh ring magnet according to the present invention is a ringmagnet which is structured in such a way that the aforementioned one endface in which the aforementioned recess is formed further has aprotrusion formed on the same end face while the aforementioned otherend face on which the aforementioned protrusion is formed further has arecess formed in the same end face.

According to this structure, it is possible to easily align central axesof the individual ring-shaped preliminarily molded pieces by fitting theprotrusions and the recesses formed as mentioned above to one anotherand thereby obtain a product that offers high precision in shape andprevents displacement of the individual ring-shaped preliminarily moldedpieces during transportation.

A twelfth ring magnet according to the present invention is a ringmagnet which is structured in such a way that the upper end face of theaforementioned ring-shaped preliminarily molded piece in a topmost layerand the lower end face of the aforementioned ring-shaped preliminarilymolded piece in a bottommost layer are flat surfaces on which neither ofthe aforementioned recess and protrusion is formed.

According to this structure, it is possible to maintain the ring-shapedmolded body in a stable condition during placement and transportationand shorten its axial length.

A first method of manufacturing a ring magnet according to the presentinvention includes a process of forming a plurality of radially orientedring-shaped preliminarily molded pieces, a process of forming aring-shaped molded body by stacking the aforementioned ring-shapedpreliminarily molded pieces in an axial direction, and a process ofsintering the aforementioned ring-shaped molded body.

According to this method, it is possible to manufacture ring magnetswhich are less susceptible to deterioration of magnetic properties inboundary regions with high productivity.

A second method of manufacturing a ring magnet according to the presentinvention is a method in which the aforementioned ring-shaped moldedbody is pressurized at a pressure of 50 MPa or less in the direction inwhich the aforementioned ring-shaped molded body has been stacked.

According to this method, it is possible to ensure close contact betweenthe ring-shaped preliminarily molded pieces.

A third method of manufacturing a ring magnet according to the presentinvention is a method in which the process of forming the cylindricalmolded body and the process of forming the ring-shaped molded bodysuccessively transfer a plurality of transferable metal die units toindividual processes mentioned below in a manner that allows executionof the aforementioned individual processes, each of the aforementionedtransferable metal die units including a die, a core which is insertedinto the interior of the aforementioned die, forming a ringlike spacebetween the aforementioned die and the aforementioned core, a lowerpunch which closes the bottom of the aforementioned space, forming acavity into which magnetic powder is fed and filled, and an upper punchfor pressurizing the magnetic powder fed into the aforementioned cavity,the process of forming the cylindrical molded body and the process offorming the ring-shaped molded body including a process of feeding andfilling the aforementioned magnetic powder into the aforementionedcavity, a process of forming each of the aforementioned ring-shapedpreliminarily molded pieces by pressurizing the aforementioned magneticpowder in the axial direction while applying a radially orientingmagnetic field to the interior of the aforementioned cavity, a processof drawing out each of the aforementioned ring-shaped preliminarilymolded pieces from the aforementioned transferable metal die unit, and aprocess of stacking in multiple layers the ring-shaped preliminarilymolded pieces which have been drawn out of the aforementionedtransferable metal die unit in the axial direction of the aforementionedring-shaped preliminarily molded pieces.

According to this method, it is possible to shorten tact time andmanufacture ring magnets which are less susceptible to deterioration ofmagnetic properties in boundary regions between the adjacent ring-shapedpreliminarily molded pieces with high productivity.

A fourth method of manufacturing a ring magnet according to the presentinvention is a method including a process of weighing a specifiedquantity of the magnetic powder to be fed into the aforementionedcavity.

According to this method, the height of the ring-shaped preliminarilymolded pieces becomes constant and it becomes possible to preventunwanted force and impact from being exerted on the ring-shapedpreliminarily molded pieces during the stacking process thereof.

A fifth method of manufacturing a ring magnet according to the presentinvention is a method including a process of moving the aforementionedupper punch to a location above the aforementioned cavity and insertingthe aforementioned upper punch into the aforementioned cavity.

According to this method, it is possible to set the transferable metaldie unit in a condition where the magnetic powder in the cavity can bepressurized by the upper punch after feeding and filling the magneticpowder in the cavity.

Also, as it is possible to form the magnetic powder by pressure moldingin a condition in which the upper punch is inserted into the cavity,there is no need for a drive mechanism for positioning individual partsof the metal die unit with high precision during pressure molding.

A sixth method of manufacturing a ring magnet according to the presentinvention is a method in which each of the aforementioned ring-shapedpreliminarily molded pieces is drawn out of the aforementioned die whilebeing pressurized in process of drawing out the aforementionedring-shaped preliminarily molded piece from the aforementionedtransferable metal die unit.

According to this method, a difference in internal stresses between anupper portion of the ring-shaped preliminarily molded piece drawn out ofthe transferable metal die unit and a lower portion of the ring-shapedpreliminarily molded piece still remaining in the transferable metal dieunit in the process of drawing out the ring-shaped preliminarily moldedpiece from the transferable metal die unit is so small that theoccurrence of cracks is prevented.

A seventh method of manufacturing a ring magnet according to the presentinvention is a method in which the aforementioned magnetic powderadhering to each of the aforementioned ring-shaped preliminarily moldedpieces is removed halfway in the process of drawing out theaforementioned ring-shaped preliminarily molded piece from theaforementioned transferable metal die unit.

According to this method, it is possible to prevent the ring-shapedpreliminarily molded piece from listing due to gaps which may occurbetween adjacent ring-shaped preliminarily molded pieces and from beingdamaged by pressurization.

An eighth method of manufacturing a ring magnet according to the presentinvention is a method in which the aforementioned ring-shapedpreliminarily molded pieces are stacked with the aforementionedring-shaped preliminarily molded pieces successively turned aboutcentral axes thereof in the process of stacking in multiple layers thering-shaped preliminarily molded pieces which have been drawn out of theaforementioned transferable metal die unit in the axial direction of theaforementioned ring-shaped preliminarily molded pieces.

According to this method, it is possible to obtain a ring magnet inwhich ridges on the ring-shaped preliminarily molded pieces havingfurrows and the ridges are turned by specific skew angles from one layerto next.

Also, if there occurs a deviation of magnetic properties in acircumferential direction between one ring-shaped preliminarily moldedpiece and another, it is possible to cancel out this deviation ofmagnetic properties.

A ninth method of manufacturing a ring magnet according to the presentinvention is a method in which the aforementioned ring-shapedpreliminarily molded pieces are stacked with top and bottom axial endfaces of the aforementioned ring-shaped preliminarily molded piece atthe time of molding turned upside down in at least one layer in theprocess of stacking in multiple layers the ring-shaped preliminarilymolded pieces which have been drawn out of the aforementionedtransferable metal die unit in the axial direction of the aforementionedring-shaped preliminarily molded pieces.

According to this method, it is possible to prevent sharp changes inmagnetic properties at joint areas of individual layers even when agradient occurs in the magnetic properties of the ring-shapedpreliminarily molded pieces between upper punch and lower punch sides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and 1(b) are perspective views showing examples of ringmagnets obtained by forming and sintering the same by a permanent magnetmolding apparatus of the present invention;

FIG. 2 is a plan view showing the structure of a magnet moldingapparatus according to a first embodiment of this invention;

FIGS. 3( a) and 3(b) show the structure of a transferable metal die unitshown in FIG. 2, in which FIG. 3( a) is a plan view and FIG. 3( b) is across-sectional view taken along a line IIIb-IIIb of FIG. 3( a);

FIGS. 4( a)-4(c) are cross-sectional views for explaining a powderfeeding/filling unit and the working thereof;

FIGS. 5( a)-5(d) are cross-sectional views for explaining the structureand working of a punch setup unit;

FIGS. 6( a)-6(c) are cross-sectional views for explaining the structureand working of a magnetic field-assisted molding unit;

FIGS. 7( a) and 7(b) are cross-sectional views showing the structure ofa pressing element;

FIGS. 8( a)-8(d) are plan views of which FIGS. 8( a) and 8(c)cross-sectional views, FIG. 8( b) is taken along a line VIIIb-VIIIb ofFIG. 8( a), and FIG. 8( d) is taken along a line VIIId-VIIId of FIG. 8(c), illustrating the structure of a back core;

FIG. 9 is a cross-sectional view showing a state of magnetic fluxesduring radial magnetic orientation;

FIG. 10( a) is a plan view and in FIG. 10( b) a cross-sectional viewtaken along a line Xb-Xb of FIG. 10( a), illustrating the structure of amold-release unit;

FIGS. 11( a)-11(d) are cross-sectional views for explaining the workingof the mold-release unit;

FIGS. 12( a) and 12(b) are cross-sectional views for explaining thestructure and working of a molded piece powder removal unit;

FIGS. 13( a) and 13(b) are cross-sectional views for explaining thestructure and working of the molded piece powder removal unit;

FIGS. 14( a) and 14(b) are cross-sectional views for explaining thestructure and working of a stacking unit;

FIGS. 15( a) and 15(b) are cross-sectional views for explaining thestructure and working of the stacking unit;

FIGS. 16( a) and 16(b) are cross-sectional views for explaining thestructure and working of the stacking unit;

FIGS. 17( a)-17(d) are diagrams showing a process of stacking moldedpieces in such a manner that the molded pieces in individual layers areskewed by desired angles;

FIG. 18 is a cross-sectional view showing a state in which thering-shaped preliminarily molded pieces are stacked in such a mannerthat axial end faces of the ring-shaped preliminarily molded piece areturned upside down;

FIGS. 19( a) and 19(b) are a perspective view and a cross-sectional viewshowing a ring-shaped molded body according to a second embodiment ofthe present invention;

FIGS. 20( a) and 20(b) are a plan view showing a ring-shapedpreliminarily molded piece and a cross-sectional view;

FIG. 21 is a cross-sectional view for explaining a magnetization moldingmethod applied to the ring-shaped preliminarily molded piece;

FIGS. 22( a)-22(e) are a plan view, FIG. 22( a), and a cross-sectionalview, FIG. 22( b), taken along a line XXIIb-XXIIb of FIG. 22( a),showing a ring-shaped molded body having a shape of a modified form ofthe second embodiment of the present invention, and cross-sectionalviews, FIG. 22( c), FIG. 22( d), and FIG. 22( e), showing ring-shapedpreliminarily molded pieces;

FIGS. 23( a)-23(c) are a plan view, FIG. 22( a), and a cross-sectionalview, FIG. 23( b), taken along a line XXIIIb-XXIIIb of FIG. 23( a)showing a ring-shaped preliminarily molded piece and a cross-sectionalview, FIG. 23( c), showing a ring-shaped molded body according to athird embodiment of the present invention;

FIG. 24 is a cross-sectional view showing a modified form of thering-shaped molded body of the third embodiment of this invention;

FIGS. 25( a)-25(c) are a plan view, FIG. 25( a), and a cross-sectionalview, FIG. 25( b), taken along a line XVb-XVb of FIG. 25( a), showing aring-shaped preliminarily molded piece, and a cross-sectional view, FIG.25( c), showing a ring-shaped molded body according to a fourthembodiment of the present invention;

FIG. 26 is a cross-sectional view showing a modified form of thering-shaped molded body of the fourth embodiment of this invention;

FIGS. 27( a)-27(c) are a plan view, FIG. 27( a), and a cross-sectionalview, FIG. 27( b), taken along a line XVIIb-XVIIb of FIG. 27( a),showing a ring-shaped preliminarily molded piece and a cross-sectionalview, FIG. 27( c), showing a ring-shaped molded body according to afifth embodiment of the present invention;

FIG. 28 is a cross-sectional view showing a modified form of thering-shaped molded body of the fifth embodiment of this invention;

FIGS. 29( a)-29(c), are a plan view, FIG. 29( a), and a cross-sectionalview, FIG. 29( b), taken along a line XXIXb-XXIXb of FIG. 29( b),showing a ring-shaped preliminarily molded piece and a cross-sectionalview, FIG. 29( c), showing a ring-shaped molded body according to asixth embodiment of the present invention;

FIG. 30 is a cross-sectional view showing a modified form of thering-shaped molded body of the sixth embodiment of this invention;

FIGS. 31( a)-31(c) are a plan view, FIG. 31( a), and a cross-sectionalview, FIG. 31( b), taken along a line XXXIb-XXIb of FIG. 31( a), showinga ring-shaped preliminarily molded piece and a cross-sectional view,FIG. 31( c), showing a ring-shaped molded body according to a seventhembodiment of the present invention;

FIG. 32 is a cross-sectional view showing a modified form of thering-shaped molded body of the seventh embodiment of this invention;

FIGS. 33( a)-33(c) are a plan view, FIG. 33( a), and a side view, FIG.33( b), showing a ring-shaped preliminarily molded piece and a sideview, FIG. 33( c), showing a ring-shaped molded body according to aneighth embodiment of the present invention;

FIG. 34 is a side view showing a modified form of the ring-shaped moldedbody of the eighth embodiment of this invention;

FIGS. 35( a)-35(c) are a plan view, FIG. 35( a), and a side view, FIG.35( b), showing a ring-shaped preliminarily molded piece and a sideview, FIG. 35( c), showing a ring-shaped molded body according to aninth embodiment of the present invention;

FIG. 36 is a side view showing a modified form of the ring-shaped moldedbody of the ninth embodiment of this invention;

FIGS. 37( a)-37(d) are a plan view, FIG. 37( a) and three side views,FIGS. 37( b)-37(d), showing a ring-shaped preliminarily molded piece anda ring-shaped molded body according to a tenth embodiment of the presentinvention;

FIG. 38 is a side view showing a modified form of the ring-shaped moldedbody of the tenth embodiment of this invention;

FIGS. 39( a)-39(d) are a plan view, and three side views, showing aring-shaped preliminarily molded piece and showing a ring-shaped moldedbody according to an eleventh embodiment of the present invention; and

FIG. 40 is a side view showing a modified form of the ring-shaped moldedbody of the eleventh embodiment of this invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Individual embodiments of the present invention are now described in thefollowing based on the drawings.

First Embodiment

FIGS. 1( a) and 1(b) are perspective views showing examples of ringmagnets obtained by forming and sintering the same by a method ofmanufacturing the ring magnets of the present invention, FIG. 2 is aplan view showing the structure of a magnet molding apparatus used forcarrying out the method of manufacturing the ring magnets according to afirst embodiment of the present invention, and FIGS. 3( a) and 3(b) showthe structure of a transferable metal die unit shown in FIG. 2. FIG. 3(a) is a plan view and FIG. 3( b) is a cross-sectional view taken alongline IIIb-IIIb of FIG. 3( a).

As illustrated in FIGS. 1( a) and 1(b), a ring magnet 1 molded andmanufactured by the magnet molding apparatus of this invention is aproduct obtained by stacking and sintering preliminarily molded ringpieces 1 a each formed into a cylindrical shape as shown in FIG. 1( a)or by stacking and sintering ring-shaped preliminarily molded pieces 1 aeach having furrows and ridges on a curved peripheral surface as shownin FIG. 1( b). (Although furrows and ridges are formed on a curved outersurface in the Figure, the ring-shaped preliminarily molded pieces 1 ainclude those having furrows and ridges also on a curved inner surface.)

The ring magnet 1 thus produced by stacking and sintering thering-shaped preliminarily molded pieces 1 a each having a short axiallength is less susceptible to deterioration of magnetic properties inboundary regions between the adjacent ring-shaped preliminarily moldedpieces 1 a. It is therefore possible to produce the ring magnet 1 havinga large total amount of magnetic flux.

As illustrated in FIG. 2, the magnet molding apparatus of thisembodiment includes a belt conveyor 2 for transferring the transferablemetal die unit 10, a powder feeding/filling unit 3 for weighing magneticpowder and feeding and filling the same into a ring-shaped cavity in thetransferable metal die unit 10, a punch setup unit 4 for setting anupper punch in a condition where the upper punch for pressurizing themagnetic powder filled in the cavity in the transferable metal die unit10, in which the magnetic powder has been filled, can form the magneticpowder by pressure molding, a magnetic field-assisted molding unit 5 forpressure-molding in a magnetic field the magnetic powder in thetransferable metal die unit 10 which has reached a condition in whichthe magnetic powder can undergo pressure molding with the upper punchset in position, a mold-release unit 6 for drawing a ring-shapedpreliminarily molded piece which has been pressure-molded in themagnetic field out of the transferable metal die unit 10, a molded piecepowder removal unit 7 for removing excess magnetic powder adhering tothe ring-shaped preliminarily molded piece which has been drawn out, astacking unit 8 for stacking ring-shaped preliminarily molded pieceswhich have been pressure-molded in the magnetic field, and a die powderremoval/setup unit 9 for removing the magnetic powder adhering to thetransferable metal die unit 10 and placing the transferable metal dieunit 10 in a transferable condition.

As shown in FIGS. 3( a) and 3(b), the transferable metal die unit 10includes a palette 10 a which travels on the belt conveyor 2, a holder(first holder) 10 b, 10 c for holding a lower metal die portionincluding a columnlike core 10 d, a lower punch 10 e and a die 10 f withthe core 10 d disposed at the center of the die 10 f, the lower punch 10e and the core 10 d together forming a cavity 10 h into which themagnetic powder is supplied, and an upper punch 10 g held by anotherholder (second holder) 10 j. The holder 10 b is an element made offerromagnetic material while the holder 10 c is an element made ofnonmagnetic material.

Mounting positions and directions of the palette 10 a and the holders 10b, 10 j, the holder 10 b and the lower punch 10 e, and the lower punch10 e and the die 10 f are restricted by respective positioning pinswhich constitute positioning mechanisms.

The provision of these positioning mechanisms serves to facilitate setupof metal die components (e.g., insertion into the core and the die intothe upper die) on the palette and positioning of the transferable metaldie unit 10 when it is transferred onto the magnetic field-assistedmolding unit 5.

The structure and working of each unit are now explained in thefollowing with reference to FIG. 2.

First, the transferable metal die unit 10 is transferred onto the powderfeeding/filling unit 3 by the belt conveyor 2.

FIGS. 4( a)-4(c) are cross-sectional views for explaining the powderfeeding/filling unit and the working thereof, in which shown in FIG. 4(a) is a process of weighing the magnetic powder and shown in FIGS. 4( b)and 4(c) is a process of feeding the magnetic powder into thetransferable metal die unit 10.

The powder feeding/filling unit 3 includes, as shown in FIG. 2, aweighing mechanism 3 d for weighing the magnetic powder and a transportmechanism 3 e for transferring the magnetic powder 11 weighed and pickedup into a vessel 3 c to the position of the transferable metal die unit10. Also, as shown in FIGS. 4( a)-4(c), the powder feeding/filling unitincludes a turning mechanism 3 f for turning the vessel 3 c to a slantangle, a powder feeding jig 3 a for guiding the magnetic powder 11 inthe vessel 3 c into the cavity 10 h and a vibrating mechanism 3 bincluding a vibrator element for vibrating the powder feeding jig 3 a.

When the transferable metal die unit 10 has been transferred up to thepowder feeding/filling unit, the powder feeding/filling unit places themagnetic powder 11 into the vessel 3 c while measuring a specific weightof the magnetic powder 11 by using a vibration feeder and a weighinginstrument in the magnetic powder weighing process shown in FIG. 4( a).

By weighing the magnetic powder to be accommodated in the vessel 3 c asdiscussed above, it is possible to maintain the height of ring-shapedmolded pieces to be formed constant and prevent damages in a stackingprocess as will be later described.

In the powder feeding process shown in FIGS. 4( b) and 4(c), the powderfeeding jig 3 a having a funnellike shape for guiding the magneticpowder 11 into the cavity 10 h of the transferable metal die unit 10 anda winglike jig (not shown) for stirring the magnetic powder fed into thecavity 10 h are set on the die 10 f of the transferable metal die unit10 and, then, the vessel 3 c accommodating the magnetic powder 11 ismoved up the position of the funnellike powder feeding jig 3 a, thepowder feeding jig 3 a is turned to a slant angle, and the magneticpowder 11 in the vessel 3 c is transferred into the funnellike powderfeeding jig 3 a. Further, an impact is given to the vessel 3 c by meansof a knocker to transfer the magnetic powder 11 in the vessel 3 c intothe funnellike powder feeding jig 3 a without leaving any magneticpowder 11 in the vessel 3 c. Further, the entirety of the magneticpowder on the powder feeding jig 3 a is transferred into the cavity 10 hby giving vibrations to the funnellike powder feeding jig 3 a by thevibrating mechanism 3 b and the magnetic powder 11 is uniformly filledin the cavity by lifting wings of the winglike jig while rotating thewings to mix the magnetic powder 11 in the cavity 10 h.

As the magnetic powder 11 is filled in the cavity by lifting the wingsof the winglike jig while rotating the wings to mix the magnetic powder11 in the cavity, any voids or bridges of the magnetic powder present inthe magnetic powder in the cavity are collapsed and the magnetic powderis uniformly filled in the cavity.

The transferable metal die unit filled with the magnetic powder istransferred to the punch setup unit 4 and set in place at a prescribedposition.

FIGS. 5( a)-5(d) are cross-sectional views for explaining the structureand working of the punch setup unit. As shown in these figures, thepunch setup unit includes a tong-grip lifter 4 a for catching the upperpunch 10 g and a transfer mechanism for raising and lowering thetong-grip lifter 4 a and moving the upper punch 10 g caught by thetong-grip lifter 4 a.

With this punch setup unit, it is possible to set the transferable metaldie unit in a condition where the magnetic powder in the cavity can bepressurized by the upper punch 10 g.

Since the transferable metal die unit 10 is set in the condition wherethe magnetic powder can be pressurized by inserting the upper punch 10 ginto the cavity, there is no need for a mechanism for positioningindividual parts of the transferable metal die unit 10 with highprecision for magnetizing pressure molding in a succeeding process.

When the palette 10 a has been transferred onto a stage of the punchsetup unit 4 and set in place at the prescribed position as shown inFIG. 5( a), the tong-grip lifter 4 a descends and catches the upperpunch log as shown in FIG. 5( b). The tong-grip lifter 4 a then liftsthe upper punch 10 g as shown in FIG. 5( c), moves toward a lower diesection and descends to fit the upper punch 10 g on the core 10 d asshown in FIG. 5( d). Subsequently, the tong-grip lifter 4 a releases theupper punch 10 g and the upper punch 10 g fits into the cavity. Thediameter of an upper end portion of the core 10 d is made smaller thanthe inside diameter of the cavity by 0.2 mm and tapered by 3°.Therefore, even if the positions of the palette and the tong-grip lifter4 a are offset from each other by an amount not exceeding 0.1 mm at thetime of punch insertion, there occurs no such a failure that the punch10 g can not be fitted over the core 10 d. Then, after releasing theupper punch 10 g, the tong-grip lifter 4 a ascends and moves back to itsoriginal position.

The transferable metal die unit 10 on which the upper punch 10 g hasbeen set is transported to a specified position of the magneticfield-assisted molding unit 5 by the belt conveyor 2.

FIGS. 6( a)-6(c) are cross-sectional views for explaining the structureand working of the magnetic field-assisted molding unit. FIGS. 7( a) and7(b) are cross-sectional views showing the structure of a pressingelement, and FIGS. 8( a)-8(d) are plan views, FIG. 8( a) and FIG. 8( c)are cross-sectional views FIG. 8( b) and FIG. 8( d) being taken alonglines VIIIb-VIIIb and VIIId-VIIId, illustrating the structure of a backcore. As shown in FIG. 2, the magnetic field-assisted molding unit 5 hasa transfer mechanism 5 h for transferring a metal die portion of thetransferable metal die unit 10 on which the upper punch 10 g has beenset from the pallet 10 a on the belt conveyor 2 to the magneticfield-assisted molding unit 5 and returning the metal die portion of thetransferable metal die unit 10 onto the pallet 10 a on the belt conveyor2 after magnetization molding. As shown in FIGS. 6( a)-6(c), themagnetic field-assisted-molding unit 5 is provided with electromagneticcoils 5 a (fixed to frames) for generating an orienting magnetic fieldfor magnetically orienting the magnetic powder, a compression moldingmechanism 5 b for raising and lowering the pressing element 5 c forpressing the top-side electromagnetic coil 5 a and the upper punch 10 g,a ring-shaped elastic member 5 j and back yokes 5 d which go intocontact with the die 10 f when actuated by an unillustrated aircylinder.

As shown in FIGS. 7( a) and 7(b), the pressing element 5 c is providedwith a punch pressing portion 5 e for pressing the upper punch, amovable rod 5 f which moves as if pushed into the interior of the punchpressing portion 5 e, a spring 5 g located between a rear surface of themovable rod 5 f and an inner surface of the punch pressing portion 5 efor pressing the movable rod 5 f against the core 10 d.

Also, the back yokes 5 d constitute one ferromagnetic member having asemicircular opening which fits on the outside diameter of the die 10 fas shown in FIG. 8. The back yokes 5 d disposed such that the center oftheir thickness coincides with the central position of the thickness ofthe die 10 f, moves in the direction of the die 10 f and goes intocontact therewith.

When the transferable metal die unit 10 has been transferred from thepunch setup unit 4 to the magnetic field-assisted molding unit 5 by thebelt conveyor 2, the metal die-portion is transferred from the palette10 a to a molding portion of the magnetic field-assisted molding unit 5together with the holder 10 b by the transfer mechanism 5 h (refer toFIG. 2) as shown in FIG. 6( a).

Next, as the compression molding mechanism 5 b is actuated, theelectromagnetic coil 5 a and the pressing element descend, the top- andbottom-side frames are fixed to each other by a clamping function andthe die 10 f is fixed by the ring-shaped elastic member 5 j which isattached to the bottom of the top-side frame as shown in FIG. 6( b).Then, the back yokes 5 d move from both sides of the die 10 f and gointo tight contact with the outer periphery of the die 10 f.Subsequently, a current is caused to flow through the electromagneticcoil 5 a, generating a radially orienting magnetic field, the pressingelement 5 c descends to press the upper punch 5 g (sic), and the upperpunch 5 g (sic) compression-molds the magnetic powder in the cavity asshown in FIG. 6( c), whereby a radially oriented molded piece isobtained. Compression molding pressure should be 10 to 100 MPa,preferably 10 MPa, and orienting magnetic field should be made equal toor higher than 1T.

FIG. 9 is a cross-sectional view showing a state of magnetic fluxesduring radial magnetic orientation. A magnetic field generated by thetop-side coil 5 a passes through the pressing element 5 c which is aferromagnetic member in the form of a magnetic flux and enters themovable rod 5 f which is also a ferromagnetic member, whereas a magneticfield generated by the bottom-side coil 5 a passes through the holder 10b which is a ferromagnetic member and enters the core 10 d (refer toFIG. 6). The lower punch 10 e and the upper punch 10 g are nonmagneticmembers.

As shown in FIG. 9, the magnetic fluxes indicated by arrows with brokenlines pass through the movable rod 5 f which is a ferromagnetic memberand the core 10 d and pass through the cavity 10 h of the die 10 f whichis the ferromagnetic member in radial directions thereof, creatingthereby a radially orienting magnetic field inside the cavity 10 h.

A radially oriented ring-shaped preliminarily molded piece is returnedonto the palette 10 a together with the metal die portion and the holder10 b by the transfer mechanism 5 h.

The transferable metal die unit including the ring-shaped preliminarilymolded piece is transported to a specified position on the mold-releaseunit 6 by the belt conveyor 2.

FIGS. 10( a) and 10(b) are a plan view FIG. 10( a), and across-sectional view, FIG. 10( b), taken along a line Xb-Xb of FIG. 10(a), illustrating the structure of the mold-release unit. As shown inthis Figure, the mold-release unit is provided with a molded piecepressurization mechanism including an air cylinder 6 a for pressurizingeach ring-shaped preliminarily molded piece 13 and an upper punchstopper 6 d, and a die lifting mechanism including a table 6 c forlifting the die 10 f upward and an air cylinder 6 b.

FIGS. 11( a)-11(d) are cross-sectional views for explaining the workingof the mold-release unit. As the air cylinder 6 a lifts up the pallet 10a as shown in FIG. 11( a), the upper punch 10 g goes in contact with theupper punch stopper 6 d so that the ring-shaped preliminarily moldedpiece 13 is pressurized. Pressurizing force should be 0.1 to 1 MPa.

Next, as shown in FIG. 11( b), the air cylinder 6 b is actuated, thetable 6 c lifts up the die 10 f and the ring-shaped preliminarily moldedpiece 13 is drawn out of the die 10 f.

Next, as shown in FIG. 11( c), the air cylinder 6 a lowers and thepallet 10 a lies on the belt conveyor 2. Carried by the belt conveyor 2,the pallet 10 a moves up to a position where the die 10 f is loaded onanother holder 10 j placed on the pallet 10 a when the die 10 fsupported by the table 6 c descends, and the table pressing cylinder 6 bis actuated so that the table 6 c descends, thereby placing the die 10 fon the holder 10 j as shown in FIG. 11( d).

If there is a difference in internal stresses between an upper portionof the ring-shaped preliminarily molded piece 13 drawn out of thetransferable metal die unit 10 and a lower portion of the ring-shapedpreliminarily molded piece 13 still remaining in the transferable metaldie unit 10 in the process of drawing out the ring-shaped preliminarilymolded piece 13 from the transferable metal die unit 10, cracks arelikely to develop. In this mold-release unit, however, the molded piece13 is drawn out of the die 10 f under conditions where the ring-shapedpreliminarily molded piece 13 is pressurized, and the difference ininternal stress between a top surface and a bottom surface of thering-shaped preliminarily molded piece 13 is so small that theoccurrence of cracks is prevented.

After the die 10 f has been drawn out, the transferable metal die unit10 is transported to a specified position of the molded piece powderremoval unit 7 by the belt conveyor 2.

FIGS. 12( a), 12(b), 13(a), and 13(b) are cross-sectional views forexplaining the structure and working of the molded piece powder removalunit. As shown in these Figures, the molded piece powder removal unit isprovided with a raise/lower mechanism including a table 7 a and an aircylinder 7 b for raising and lowering the table 7 a, a nozzle 7 c forspewing out nitrogen gas and a dust collecting duct 7 d for drawing andcollecting the magnetic powder into a dust collector.

As shown in FIG. 12( a), the pallet 10 a halts at the specifiedposition, the air cylinder 7 b is actuated and the table 7 a ascends.Then, as shown in FIG. 12( b), the lower punch 10 e ascends, supportedby the table 7 a, and the ring-shaped preliminarily molded piece 13 isdrawn apart from the core 10 d. At this time, the upper punch 10 g isdrawn out simultaneously and placed on another holder 10 j (refer toFIG. 3).

When the top surface of the ring-shaped preliminarily molded piece 13has slightly protruded beyond the core 10 d in the process of drawingout the ring-shaped preliminarily molded piece 13 from the core 10 d,nitrogen gas is spewed out from the nozzle 7 c to blow out the magneticpowder adhering to the surface of the ring-shaped preliminarily moldedpiece 13 and the magnetic powder is sucked up by the dust collectingduct 7 d as shown in FIG. 13( a). Subsequently, the ring-shapedpreliminarily molded piece 13 is drawn out from the core 10 d as shownin FIG. 13( b).

By removing excess magnetic powder from the ring-shaped preliminarilymolded piece 13 by the molded piece powder removal unit 7, it ispossible to prevent the ring-shaped preliminarily molded piece 13 fromlisting due to gaps which may occur between adjacent ring-shapedpreliminarily molded pieces 13 and from being damaged by pressurizationduring stacking in a succeeding process.

After the powder removal process, the ring-shaped preliminarily moldedpiece 13 is transferred together with the holder 10 b, the core 10 d andthe lower punch to the stacking unit 8 by a transfer mechanism 12.

FIGS. 14( a), 14(b), 15(a), 15(b), and 16(a), 16(b) are cross-sectionalviews for explaining the structure and working of the stacking unit. Asshown in these Figures, the stacking unit is provided with a tong-griplifter 8 a serving as a mechanism for gripping the ring-shapedpreliminarily molded piece 13, a table 8 b on which ring-shapedpreliminarily molded pieces 13 are stacked, a mechanism for positioning,raising, lowering and moving the tong-grip lifter 8 a and a turningmechanism, such as a motor, for turning the table 8 b although thelatter two mechanisms are not illustrated.

As shown in FIG. 14( a), the tong-grip lifter 8 a is moved to a locationjust above the ring-shaped preliminarily molded piece 13 drawn from thecore 10 d. Then, as shown in FIG. 14( b), the tong-grip lifter 8 a islowered to grip the ring-shaped preliminarily molded piece 13. Grippingforce is adjusted to 0.1 to 4N.

Next, the tong-grip lifter 8 a is raised and, as shown in FIG. 15( a),the tong-grip lifter 8 a is moved such that its center is locatedexactly above the center of rotation of the table 8 b. Then, as shown inFIG. 15( b), the tong-grip lifter 8 a is lowered and the ring-shapedpreliminarily molded piece 13 is placed on the table 8 b. At this time,the center of the ring-shaped preliminarily molded piece 13 coincideswith the center of rotation of the table 8 b.

Further, using the same procedure, ring-shaped preliminarily moldedpieces 13 in second and third layers are stacked on top of thering-shaped preliminarily molded piece 13 in a first layer as shown inFIGS. 16( a) and 16(b). Ring-shaped preliminarily molded pieces 13 arestacked up to a necessary number of layers by repeating this stackingprocess.

If the height of the stacked ring-shaped preliminarily molded pieces 13becomes too large due to the occurrence of variations in height amongthe individual ring-shaped preliminarily molded pieces 13, undesirablepressure will be exerted on the ring-shaped preliminarily molded pieces13 during the stacking process, potentially causing crushing of thering-shaped preliminarily molded pieces 13. If the height of the stackedring-shaped preliminarily molded pieces 13 becomes too small, thetong-grip lifter 8 a may release the ring-shaped preliminarily moldedpiece 13 in the air, potentially causing breakage of the ring-shapedpreliminarily molded piece 13 as a result of an impact of fall. In thepresent embodiment, however, the weight of the ring-shaped preliminarilymolded piece 13 molded in one cycle is measured to a fixed amount in themagnetic powder weighing process carried out by the powderfeeding/filling unit 3 shown in FIG. 2, so that the height of eachring-shaped preliminarily molded piece 13 is kept constant and therewill not arise such a problem that an undesirable force or an impactforce is exerted on the ring-shaped preliminarily molded piece 13 duringthe stacking process.

FIGS. 17( a)-17(d) are diagrams showing a process of stackingring-shaped preliminarily molded pieces 13 in such a manner that thering-shaped preliminarily molded pieces 13 in individual layers areturned, or skewed, by desired angles. For example, the ring-shapedpreliminarily molded piece 13 of the first layer is placed on the table8 b such that a position A of the ring-shaped preliminarily molded piece13 is located as shown in FIG. 17( a). Then, the table 8 b is turned by180° such that the position A of the ring-shaped preliminarily moldedpiece 13 is located as shown in FIG. 17( b), and the ring-shapedpreliminarily molded piece 13 of the second layer is stacked. Thering-shaped preliminarily molded piece 13 of the third layer is stackedafter further turning the table 8 b by 90° as shown in FIG. 17( c), andthe ring-shaped preliminarily molded piece 13 of the fourth layer isstacked after further turning the table 8 b by 180° as shown in FIG. 17(d).

By stacking the ring-shaped preliminarily molded pieces 13 in theindividual layers in a manner that the preliminarily molded pieces 13are skewed by desired angles as mentioned above, it is possible toobtain a ring magnet in which ridges on the ring-shaped preliminarilymolded pieces 1 a having furrows and ridges are turned by specific skewangles from one layer to next as shown in FIG. 1( b), for example.

Also, if there occurs a deviation of magnetic properties in acircumferential direction between one ring-shaped preliminarily moldedpiece 13 and another, it is possible to cancel out this deviation ofmagnetic properties.

FIG. 18 is a cross-sectional view showing a state in which thering-shaped preliminarily molded pieces are stacked in such a mannerthat axial end faces of the ring-shaped preliminarily molded piece ofevery other layer at the time of molding are turned upside down. Incertain cases, a gradient could occur in the magnetic properties of thering-shaped preliminarily molded pieces 13 between the upper punch sideand the lower punch side, causing sharp changes in magnetic propertiesat joint areas of the individual layers, for example. It is howeverpossible to prevent such sharp changes in magnetic properties at thejoint areas of the individual layers by placing the ring-shapedpreliminarily molded piece 13 of the first layer with its axial endfaces turned upside down such that the bottom face is the upper punchside, stacking the ring-shaped preliminarily molded piece 13 of thesecond layer without turning its axial end faces upside down such thatthe bottom face becomes the lower punch side, and so on, by alternatelystacking the ring-shaped preliminarily molded piece 13 turned upsidedown and the ring-shaped preliminarily molded piece 13 unturned in asuccessive sequence as shown in FIG. 18.

Shown in FIG. 18 is a case where the ring-shaped preliminarily moldedpieces are stacked with their axial end faces at the time of moldingturned upside down every other layer. In a case where a gradient in themagnetic properties occurs in an upper layer only, or a gradient in themagnetic properties occurs in a lower layer only, the same effect asdiscussed above can be obtained by stacking the ring-shapedpreliminarily molded piece of the relevant layer only with their axialend faces at the time of molding turned upside down.

In order to stack the ring-shaped preliminarily molded pieces 13 turnedupside down as discussed above, there is provided a rotary actuator in aclamp portion of the tong-grip lifter 8 a as shown in FIGS. 14( a) to16(b).

Upon completion of the stacking process, the metal die parts 10 d, 10 eand the holder 10 b are returned onto the palette 10 a by the transfermechanism 12 and conveyed to the die powder removal/setup unit 9 where anext process is performed.

The die powder removal/setup unit 9 is provided with a powder removalmechanism for removing magnetic powder adhering to the transferablemetal die unit 10 and a setup mechanism for setting up the individualparts of the transferable metal die unit 10 in an initial conditionwhich allows the powder feeding/filling unit 3 to feed the magneticpowder.

The powder removal mechanism has a nozzle (including a mechanism formoving the nozzle to the individual parts of the transferable metal dieunit 10) which can blow nitrogen gas against the individual parts of thetransferable metal die unit 10 and a vacuum mechanism for drawing andcollecting the magnetic powder blown off by nitrogen gas.

With the provision of the powder removal mechanism and the setupmechanism, it is possible to smoothly carry out a next cycle of moldingto the staking process.

The setup mechanism is a mechanism for lifting the die 10 f positionedon the holder 10 j shown in FIGS. 3( a) and 3(b) and moving the die 10 fonto the lower punch 10 e placed on the holder 10 b upon completion ofthe stacking process.

A cylindrical molded body formed by stacking the ring-shapedpreliminarily molded pieces 13 is transferred to a sintering/heattreatment furnace. After sintering and heat treatment at a specifiedtemperature, the cylindrical molded body is subjected to finishingoperation, as necessary, whereby the ring magnet 1 show in FIG. 1 isobtained.

According to a manufacturing system for manufacturing ring magnets ofthis embodiment, a plurality of transferable metal die units 10 aretransported by use of the belt conveyor 2 and, at the same time, theprocesses of manufacturing axially short ring-shaped preliminarilymolded pieces are performed and a necessary number of ring-shapedpreliminarily molded pieces are stacked by the respective units providedat respective locations. This makes it possible to shorten tact time andmanufacture ring magnets which are less susceptible to deterioration ofmagnetic properties in boundary regions between the adjacent ring-shapedpreliminarily molded pieces with high productivity.

Second Embodiment

FIGS. 19( a) and 19(b) are a perspective view and a cross-sectional viewshowing a ring-shaped molded body formed by stacking ring-shapedpreliminarily molded pieces according to a second embodiment of thepresent invention, and FIG. 20( a) is a plan view showing one of thering-shaped preliminarily molded pieces and FIG. 20( b) is across-sectional view taken along a line XXb-XXb of FIG. 20( a).

As shown in FIGS. 19( a) and 19(b), the ring-shaped molded body 1 b isformed by stacking a plurality of (three as illustrated) ring-shapedpreliminarily molded pieces 1 a which are magnetically oriented inradial directions and joining the same into a single structure.

Also, as depicted in FIGS. 20( a) and 20(b), each of the ring-shapedpreliminarily molded pieces 1 a has a recess 1 c formed in one end faceall along the periphery thereof and a protrusion 1 e formed on the otherend face all along the periphery thereof. Since the ring-shapedpreliminarily molded pieces 1 a are joined into a single structure ateach mating part 1 d where the protrusion 1 e fits into thecorresponding recess 1 c, it is possible to easily align central axes ofthe individual ring-shaped preliminarily molded pieces 1 a.

FIG. 21 is a cross-sectional view for explaining a magnetization moldingmethod applied to the ring-shaped preliminarily molded pieces of thisembodiment. As shown in this figure, a metal die unit includes a die 10f which is a ring-shaped ferromagnetic material element made of steel orsuper-hard material, for instance, upper and lower cores 10 d made ofsteel, for instance, upper and lower punches 10 g, 10 e made ofnonmagnetic stainless steel or super-hard material, in which an extremeend of one of the upper and lower punches 10 g, 10 e has an arciformconvex shape whereas an extreme end of the other has a concave shape ofan arciform form having the same radius of curvature as the arciformconvex shape.

Also, the structure of this embodiment is such that a cavity 10 h isformed amid a curved inner surface of the die 10 f which is thering-shaped ferromagnetic material element, curved outer surfaces ofupper and lower cores 4 (sic), and the extreme end surfaces of the upperand lower punches 10 g, 10 e which are inserted between both.

There are provided annular electromagnetic coils 5 a surrounding theupper and lower cores 10 d. When currents are caused to flow through theelectromagnetic coils 5 a, there is generated an orienting magneticfield passing through the upper and lower cores 10 d and the die 10 f asindicated by arrows with broken lines.

Under conditions where the orienting magnetic field is applied, magneticpowder is fed into the cavity 10 h and pressurized by the upper andlower punches 10 g, 10 e, whereby a radially oriented ring-shapedpreliminarily molded piece 1 a is obtained, one ringlike end face of thering-shaped preliminarily molded piece 1 a being concave-shaped and theother end face being convex-shaped.

A plurality of ring-shaped preliminarily molded pieces 1 a thus obtainedare stacked with their recesses 1 c and protrusions 1 e fitted to oneanother as shown in FIGS. 19( a) and 19(b) to obtain the ring-shapedmolded body 1 b. The ring-shaped preliminarily molded pieces 1 a can bebonded into a single structure in a more reliable fashion bypressurizing them at a pressure which will not destroy the ring-shapedmolded body 1 b in an axial direction thereof, that is, a pressure of 50MPa or less, after fitting the recesses 1 c and protrusions 1 e to oneanother.

It is possible to manufacture a ring magnet having a large axial lengthby sintering the ring-shaped molded body 1 b thus obtained at aspecified temperature in a specific atmosphere and then performing aheat treatment like aging.

According to the present embodiment, the ring-shaped molded body 1 b isproduced by stacking in a separate process the ring-shaped preliminarilymolded pieces 1 a which have been individually formed in the presence ofthe magnetic field. Therefore, disturbances in radial magneticorientation, which occur in boundary regions of lamination of a priorart product produced by stacking individual molded pieces while formingthem in a magnetic field, do not occur, making it possible to obtainring magnets having good magnetic properties.

In addition, the recess 1 c is formed in one end face of eachring-shaped preliminarily molded piece 1 a while the protrusion 1 e isformed on the other end face, and a plurality of such ring-shapedpreliminarily molded pieces 1 a are assembled into a single structurewith the recesses 1 c and the protrusions 1 e fitted to one another. Itis therefore possible to easily align central axes of the individualring-shaped preliminarily molded pieces 1 a and thereby obtain a productthat offers high precision in shape and prevents displacement duringtransportation.

While the recess 1 c and the protrusion 1 e are formed at both end facesof the ring-shaped preliminarily molded pieces 1 a in the topmost andbottommost layers in FIG. 19, it is possible to decrease the overalllength of the ring-shaped molded body 1 by making the upper and lowerend faces of the ring-shaped preliminarily molded pieces 1 a in thetopmost and bottommost layers flat, respectively.

FIGS. 22( a) and 22(b) are a plan view and cross-sectional view takenalong line XXIIb-XXIIb of FIG. 22( a) showing a ring-shaped molded body1 b in which the upper and lower end faces of the ring-shapedpreliminarily molded pieces 1 a in the topmost and bottommost layers,respectively, are flattened FIGS. 22( c)-22(e) are cross-sectional viewsof the ring-shaped preliminarily molded pieces 1 a. As depicted in thefigures, the upper end face of the ring-shaped preliminarily moldedpiece 1 a in the topmost layer (c) is made flat, the recess 1 c and theprotrusion 1 e are formed on the upper and lower end faces of thering-shaped preliminarily molded piece 1 a in a middle layer (d),respectively, and the lower end face of the ring-shaped preliminarilymolded piece 1 a in the bottommost layer (e) is made flat.

It is possible to shorten the axial length and stabilize installationand transport conditions thereof by making the upper and lower end facesof the ring-shaped molded body 1 b flat in a manner discussed above.

Third Embodiment

FIG. 23( a) is a plan view and FIGS. 23( b) and 23(c) arecross-sectional views taken along line XXIII(b)-XXXIII(b) of FIG. 23(a). FIG. 23( b) shows a ring-shaped preliminarily molded piece and FIG.23( c) shows a ring-shaped molded body according to a third embodimentof the present invention. FIG. 24 is a cross-sectional view showing amodified form of the ring-shaped molded body of the third embodiment ofthis invention, in which the same reference numerals as used in FIGS.19( a) and 19(b) indicate portions identical or corresponding to thoseof FIGS. 19( a) and 19(b).

While the recesses and the protrusions are arc-shaped in cross sectionin the aforementioned second embodiment, recesses 1 c and protrusions 1e of the ring-shaped preliminarily molded pieces 1 a are V-shaped incross section as shown in FIGS. 23( a) and 23(b) in the presentembodiment, and the ring-shaped molded body 1 b is manufactured byfitting the recesses 1 c and the protrusions 1 e of a plurality ofring-shaped preliminarily molded pieces 1 a to one another at matingparts 1 d as shown in FIG. 23( c).

Also, it is possible to shorten the overall length of the ring-shapedmolded body 1 b and stabilize installation and transport conditionsthereof by making an upper end face of the ring-shaped preliminarilymolded piece 1 a in the topmost layer and a lower end face of thering-shaped preliminarily molded piece 1 a in the bottommost layer flatas shown in FIG. 24.

Fourth Embodiment

Given in FIGS. 25( a)-25(c) are a plan view, FIG. 25( a), andcross-sectional views taken along line XXVb-XXVB in FIG. 25( a), FIG.25( b) showing a ring-shaped preliminarily molded piece and FIG. 25( c)showing a ring-shaped molded body according to a fourth embodiment ofthe present invention. FIG. 26 is a cross-sectional view showing amodified form of the ring-shaped molded body of the fourth embodiment ofthis invention, in which the same reference numerals as used in FIGS.19( a) and 19(b) indicate portions identical or corresponding to thoseof FIGS. 19( a) and 19(b).

In this embodiment, one end face of the ring-shaped preliminarily moldedpiece 1 a is shaped into a slant surface sloping down toward an innerperiphery to form a recess 1 c and the other end face of the ring-shapedpreliminarily molded piece 1 a is shaped into a slant surface having thesame angle of inclination as the recess 1 c sloping down toward an outerperiphery to form a protrusion 1 e as shown in FIGS. 25( a) and 25(b),and the ring-shaped molded body 1 b is manufactured by fitting therecesses 1 c and the protrusions 1 e of a plurality of ring-shapedpreliminarily molded pieces 1 a to one another at mating parts 1 d asshown in FIG. 25( c).

Also, it is possible to shorten the overall length of the ring-shapedmolded body 1 b and stabilize installation and transport conditionsthereof by making the upper end face of the ring-shaped preliminarilymolded piece 1 a in the topmost layer and the lower end face of thering-shaped preliminarily molded piece 1 a in the bottommost layer flatas shown in FIG. 26.

Fifth Embodiment

FIGS. 27( a)-27(c) are a plan view, FIG. 27( a), and cross-sectionalviews taken along line XXVIIb-XXVIIb, FIG. 27( b) showing a ring-shapedpreliminarily molded piece and FIG. 27( c) showing a ring-shaped moldedbody according to a fifth embodiment of the present invention, and FIG.28 is a cross-sectional view showing a modified form of the ring-shapedmolded body of the fifth embodiment of this invention, in which the samereference numerals as used in FIGS. 19( a) and 19(b) indicate portionsidentical or corresponding to those of FIGS. 19( a) and 19(b).

In this embodiment, a ringlike groove arc-shaped in cross section isformed along the circumference in one end face of the ring-shapedpreliminarily molded piece 1 a to form a recess 1 c while an arc-shapedprojecting part having the same shape as the arc shape of the recess 1 cis formed on the other end face of the ring-shaped preliminarily moldedpiece 1 a to form a protrusion 1 e as shown in FIGS. 27( a) and 27(b),and the ring-shaped molded body 1 b is manufactured by fitting therecesses 1 c and the protrusions 1 e of a plurality of ring-shapedpreliminarily molded pieces 1 a to one another at mating parts 1 d asshown in FIG. 27( c).

Also, it is possible to shorten the overall length of the ring-shapedmolded body 1 b and stabilize installation and transport conditionsthereof by making the upper end face of the ring-shaped preliminarilymolded piece 1 a in the topmost layer and the lower end face of thering-shaped preliminarily molded piece 1 a in the bottommost layer flatas shown in FIG. 28.

In addition, the cross-sectional shape of the recesses 1 c and theprotrusions 1 e is not limited to the arc-shaped form but may beV-shaped, trapezoidal or U-shaped.

Sixth Embodiment

FIGS. 29( a)-29(c) are a plan view, FIG. 29( a), and cross-sectionalviews taken along line XXIXb-XXIXb in FIG. 29( a), FIG. 29( b) showing aring-shaped preliminarily molded piece and FIG. 29( c) showing aring-shaped molded body according to a sixth embodiment of the presentinvention, and FIG. 30 is a cross-sectional view showing a modified formof the ring-shaped molded body of the sixth embodiment of thisinvention, in which the same reference numerals as used in FIGS. 19( a)and 19(b) indicate portions identical or corresponding to those of FIGS.19( a) and 19(b).

In this embodiment, there is made a stepped structure between an innerperipheral part and an outer peripheral part of both end faces of thering-shaped preliminarily molded piece 1 a with a recess 1 c formed atthe inner peripheral part of one end face and a protrusion 1 e formed atthe inner peripheral part of the other end face as shown in FIGS. 29( a)and 29(b), and the ring-shaped molded body 1 b is manufactured byfitting the recesses 1 c and the protrusions 1 e of a plurality ofring-shaped preliminarily molded pieces 1 a to one another at matingparts 1 d as shown in FIG. 29( c).

Although not illustrated, it is possible to easily fit the recesses andthe protrusions of the preliminarily molded pieces to one another withprovision of a stepped structure having a tapered shape.

Also, it is possible to shorten the overall length of the ring-shapedmolded body 1 b and stabilize installation and transport conditionsthereof by making the upper end face of the ring-shaped preliminarilymolded piece 1 a in the topmost layer and the lower end face of thering-shaped preliminarily molded piece 1 a in the bottommost layer flatas shown in FIG. 30.

While the foregoing discussion of the first to sixth embodiments hasillustrated cases in which the recess 1 c and the protrusion 1 e areformed in a ring-shaped pattern all along the circumference of thering-shaped preliminarily molded piece 1 a, either the recess 1 c or theprotrusion 1 e may be formed in a broken pattern.

Seventh Embodiment

FIGS. 31( a)-31(c) are a plan view, FIG. 31( a), and cross-sectionalviews taken along line XXXIb-XXXIb of FIG. 31( a), FIG. 31( b) showing aring-shaped preliminarily molded piece and FIG. 31( c) showing aring-shaped molded body according to a seventh embodiment of the presentinvention. FIG. 32 is a cross-sectional view showing a modified form ofthe ring-shaped molded body of the seventh embodiment of this invention,in which the same reference numerals as used in FIGS. 19( a) and 19(b)indicate portions identical or corresponding to those of FIGS. 19( a)and 19(b).

In this embodiment, there are formed four generally hemisphericalrecesses 1 c in one end face of the ring-shaped preliminarily moldedpiece 1 a while there are made four projecting parts having the samegenerally hemispherical shape on the other end face of the ring-shapedpreliminarily molded piece 1 a to form protrusions 1 e as shown in FIGS.31( a) and 31(b), and the ring-shaped molded body 1 b is manufactured byfitting the recesses 1 c and the protrusions 1 e of a plurality ofring-shaped preliminarily molded pieces 1 a to one another at matingparts 1 d as shown in FIG. 31( c).

Although the example of FIG. 31 illustrates a case where there areprovided four each recesses 1 c and protrusions 1 e, the number of therecesses 1 c and the protrusions 1 e is not limited to four each but maybe two or more each.

Also, it is possible to shorten the overall length of the ring-shapedmolded body 1 b and stabilize installation and transport conditionsthereof by making the upper end face of the ring-shaped preliminarilymolded piece 1 a in the topmost layer and the lower end face of thering-shaped preliminarily molded piece 1 a in the bottommost layer flatas shown in FIG. 32.

Eighth Embodiment

FIGS. 33( a)-33(c) are a plan view, FIG. 33( a), a side view, FIG. 33(b), showing a ring-shaped preliminarily molded piece, and a side view,FIG. 33( c), showing a ring-shaped molded body according to an eighthembodiment of the present invention. FIG. 34 is a side view showing amodified form of the ring-shaped molded body of the eighth embodiment ofthis invention, in which the same reference numerals as used in FIGS.19( a) and 19(b) indicate portions identical or corresponding to thoseof FIGS. 19( a) and 19(b).

In this embodiment, there are formed four recesses 1 c, having agenerally semicircular shape in cross section, extending in radialdirections at 90° intervals in one end face of the ring-shapedpreliminarily molded piece 1 a while there are formed four semicircularprotrusions 1 e having the same generally semicircular shape in crosssection, extending in the radial directions on the other end face of thering-shaped preliminarily molded piece 1 a as shown in FIGS. 33( a) and31(b), and the ring-shaped molded body 1 b is manufactured by fittingthe recesses 1 c and the protrusions 1 e of a plurality of ring-shapedpreliminarily molded pieces 1 a to one another at mating parts 1 d asshown in FIG. 33( c).

Although the example of FIG. 33 illustrates a case where there areprovided four each recesses 1 c and protrusions 1 e, the number of therecesses 1 c and the protrusions 1 e is not limited to four each but maybe two or more each.

Also, it is possible to shorten the overall length of the ring-shapedmolded body 1 b and stabilize installation and transport conditionsthereof by making the upper end face of the ring-shaped preliminarilymolded piece 1 a in the topmost layer and the lower end face of thering-shaped preliminarily molded piece 1 a in the bottommost layer flatas shown in FIG. 34.

Ninth Embodiment

FIGS. 35( a)-35(e) are a plan view, FIG. 35( a), a side view, FIG. 35(b), showing a ring-shaped preliminarily molded piece and a side view,FIG. 35( c), showing a ring-shaped molded body according to a ninthembodiment of the present invention. FIG. 36 is a side view showing amodified form of the ring-shaped molded body of the ninth embodiment ofthis invention, in which the same reference numerals as used in FIGS.19( a) and 19(b) indicate portions identical or corresponding to thoseof FIGS. 19( a) and 19(b).

The present embodiment is characterized in that each of the protrusions1 e and the recesses 1 c of the foregoing eighth embodiment is reshapedto have, in a radially extending direction, a trapezoidal shape, incross section, as shown in FIGS. 35( a) and 35(b).

While the example of FIGS. 35( a)-35(c) illustrates a case where fourrecesses 1 c and protrusions 1 e are provided at 90° intervals, thenumber of the recesses 1 c and the protrusions 1 e is not limited tofour each but may be two or more each.

Also, it is possible to shorten the overall length of the ring-shapedmolded body 1 b and stabilize installation and transport conditionsthereof by making the upper end face of the ring-shaped preliminarilymolded piece 1 a in the topmost layer and the lower end face of thering-shaped preliminarily molded piece 1 a in the bottommost layer flatas shown in FIG. 36.

Tenth Embodiment

FIGS. 37( a)-37(d) are a plan view, FIG. 37( a), side views, FIGS. 37(b) and 37(c), showing a ring-shaped preliminarily molded piece and aside view, FIG. 37( d), showing a ring-shaped molded body according to atenth embodiment of the present invention. FIG. 38 is a side viewshowing a modified form of the ring-shaped molded body of the tenthembodiment of this invention, in which the same reference numerals asused in FIGS. 19( a) and 19(b) indicate portions identical orcorresponding to those of FIGS. 19( a) and 19(b).

In this embodiment, there are alternately formed semicircularprotrusions 1 e and recesses 1 c having a generally semicircular shapein cross section, extending in radial directions at 90° intervals in oneend face of the ring-shaped preliminarily molded piece 1 a while thereare alternately formed semicircular recesses 1 c and protrusions 1 ehaving the same generally semicircular shape, in cross section, at 90°intervals on the other end face of the ring-shaped preliminarily moldedpiece 1 a as shown in FIGS. 37( a), 37(b), and 37(c), and thering-shaped molded body 1 b is manufactured by fitting the recesses 1 cand the protrusions 1 e of a plurality of ring-shaped preliminarilymolded pieces 1 a to one another at mating parts 1 d as shown in FIG.37( d).

Although the example of FIGS. 37( a)-37(d) illustrates a case wherethere are provided two each recesses 1 c and protrusions 1 e, the numberof the recesses 1 c and the protrusions 1 e is not limited to two eachbut may be one or more each.

Also, it is possible to shorten the overall length of the ring-shapedmolded body 1 b and stabilize installation and transport conditionsthereof by making the upper end face of the ring-shaped preliminarilymolded piece 1 a in the topmost layer and the lower end face of thering-shaped preliminarily molded piece 1 a in the bottommost layer flatas shown in FIG. 38.

According to this embodiment, it becomes easier to produce a metal diefor molding the ring-shaped preliminarily molded pieces 1 a.

Eleventh Embodiment

FIGS. 39( a)-39(d) are a plan view, FIG. 39( a), and side views, FIGS.39( b) and 39(c), showing a ring-shaped preliminarily molded piece and aside view, FIG. 39( d), showing a ring-shaped molded body according toan eleventh embodiment of the present invention. FIG. 40 is a side viewshowing a modified form of the ring-shaped molded body of the eleventhembodiment of this invention, in which the same reference numerals asused in FIGS. 19( a) and 19(b) indicate portions identical orcorresponding to those of FIGS. 19( a) and 19(b).

The present embodiment is characterized in that each of the protrusions1 e and the recesses 1 c of the foregoing ninth embodiment is reshapedinto a radially extending trapezoidal shape, in cross section, as shownin FIGS. 39( a), 39(b), and 39(c), the protrusions 1 e and the recesses1 c being alternately formed at 90° intervals.

Although the example of FIGS. 39( a)-39(d) illustrates a case wherethere are provided two each recesses 1 c and protrusions 1 e, the numberof the recesses 1 c and the protrusions 1 e is not limited to two eachbut may be one or more each.

Also, it is possible to shorten the overall length of the ring-shapedmolded body 1 b and stabilize installation and transport conditionsthereof by making the upper end face of the ring-shaped preliminarilymolded piece 1 a in the topmost layer and the lower end face of thering-shaped preliminarily molded piece 1 a in the bottommost layer flatas shown in FIG. 40.

In addition, while the foregoing eighth to eleventh embodiments haveillustrated cases where the cross-sectional shapes of the recesses 1 cand the protrusions 1 e are generally semicircular or trapezoidal, theinvention is not limited thereto but the recesses 1 c and theprotrusions 1 e may be V-shaped, trapezoidal or U-shaped.

According to the aforementioned third to eleventh embodiments, it ispossible to make ring magnets free of disturbances in radial magneticorientation and having good magnetic properties, the ring magnetsoffering high precision in shape and a capability to preventdisplacement of ring-shaped preliminarily molded pieces duringtransportation, as in the foregoing second embodiment.

While various kinds of recesses 1 c formed in one end face of aring-shaped preliminarily molded piece 1 a and protrusions 1 e formed onthe other end face have thus far been illustrated with respect to theirshapes in the aforementioned second to eleventh embodiments, the presentinvention is not limited to those shapes. What is essential for thestructure of this invention is that a recess 1 c and a protrusion 1 ethat fits into the recess 1 c are provided in one end face and the otherend face of a ring-shaped preliminarily molded piece 1 a, respectively,the recesses 1 c and the protrusions 1 e of a plurality of preliminarilymolded pieces 2 (sic) are fitted together such that relative movementsin radial directions of the ring-shaped preliminarily molded pieces 1 astacked are constrained.

The ring magnets of this invention are applicable also to a method ofstacking ring-shaped preliminarily molded pieces while forming the samein the presence of a magnetic field in a metal die.

To add, raw material of the ring-shaped molded body 1 b may be powder ofa magnetic substance, such as Nd₂Fe₁₄B, as well as a bondable magneticmaterial made by mixing a resin into magnetic powder.

INDUSTRIAL APPLICABILITY

The present invention is for use in manufacturing permanent magnets usedin rotating electric machines, such as motors.

1. A ring magnet comprising a plurality of ring-shaped preliminarilymolded pieces, stacked in an axial direction of the molded pieces toform a tubular body for sintering, wherein each ring-shapedpreliminarily molded piece includes first and second generally annularend faces that are transverse to the axial direction of each of thering-shaped preliminarily molded pieces, each of the first end facesincludes a plurality of recesses that are spaced apart along acircumferential direction of the first end faces, each of the second endfaces includes a plurality of protrusions that are spaced apart along acircumferential direction of the second end faces and that arecomplementary in shape and location to the recesses on the first endfaces, and the recesses and the protrusions of the ring-shapedpreliminarily molded pieces adjacent to each other in the tubular bodyare engaged with each other to establish and maintain, with respect torotation about the axial direction and translation transverse to theaxial direction, relative rotational orientation and axial alignmentbetween the ring-shaped preliminarily molded pieces in the tubular body.2. The ring magnet as recited in claim 1, wherein the recesses and theprotrusions include a plurality of spherical recesses and protrusions onthe first and second end surfaces.
 3. The ring magnet as recited inclaim 1, wherein the recesses and the protrusions include extend inradial directions on the first and second end faces, respectively. 4.The ring magnet as recited in claim 3, wherein each of first and secondannular end faces is defined by inner and outer edges, and the recessesand protrusions intersect the inner and outer edges of the first andsecond annular end faces.
 5. The ring magnet as recited in claim 1,wherein the first end face includes a protrusion and the second end faceincludes a recess complementary in shape and position to the protrusionof said first end face.
 6. The ring magnet as recited in claim 1,further including a ring-shaped preliminarily molded piece in a topmostlayer of the tubular body and having an upper end face and a lowerring-shaped preliminarily molded piece in a bottommost layer of thetubular body and having a lower end face, wherein the upper end face andthe lower end face are flat annular surfaces on which neither a recessnor a protrusion is present.
 7. The ring magnet as recited in claim 1,wherein each of the first end faces includes at least three recesses andeach of the second end faces includes at least three protrusions.
 8. Thering magnet as recited in claim 1, wherein each of the first and secondannular end faces is defined by inner and outer edges of the first andsecond end faces, and the recesses and the protrusions are spaced fromthe inner and outer edges of the first and second end faces.
 9. A methodof manufacturing a ring magnet, the method comprising: forming aplurality of ring-shaped preliminarily molded pieces, each preliminarilymolded piece including opposed first and second generally annular endfaces, transverse to an axial direction of the ring-shaped preliminarilymolded pieces, each of the first end faces including a plurality ofrecesses that are spaced apart along a circumferential direction of thefirst end faces, and each of the second end faces including a pluralityof protrusions spaced apart along a circumferential direction of thesecond end faces, complementary in shape and location to the recesses onthe first end faces; forming a tubular body by stacking the ring-shapedpreliminarily molded pieces in the axial direction of the ring-shapedpreliminarily molded pieces with said recesses and protrusions ofadjacent ring-shaped preliminarily molded pieces engaged to establishand maintain, with respect to rotation about the axial direction andtranslation transverse to the axial direction, relative rotationalorientation and axial alignment between the ring-shaped preliminarilymolded pieces in the tubular body; and sintering the tubular body. 10.The method of manufacturing a ring magnet as recited in claim 9,including pressurizing the ring-shaped preliminarily molded pieces at apressure of up to 50 MPa along the axial direction in which thering-shaped preliminarily molded pieces have been stacked.
 11. Themethod of manufacturing a ring magnet as recited in claim 9, includingforming at least three recesses and at least three protrusions in thefirst and second end faces, respectively.
 12. The method ofmanufacturing a ring magnet as recited in claim 9, wherein the first andsecond annular end faces are defined by inner and outer edges of thefirst and second end faces, and the recesses and the protrusions arespaced from the first and second edges.
 13. The method of manufacturinga ring magnet as recited in claim 9, wherein the first and secondannular end faces are defined by inner and outer edges of the first andsecond end faces, and the recesses and the protrusions are radial andintersect the inner and outer edges of the first and second end faces.14. The method of manufacturing a ring magnet as recited in claim 9,including successively transferring a plurality of transferable metaldie units for execution of individual processes, each of thetransferable metal die units including a die, a core which is insertedinto the die to form a ring-like space between the die and the core, alower punch which closes the bottom of the ring-like space and forms acavity into which magnetic powder is fed to fill the cavity, and anupper punch for pressurizing the magnetic powder fed into the cavity,the method including: filling the cavity with magnetic powder;pressurizing the magnetic powder in the axial direction, while applyinga radially orienting magnetic field to the cavity to form each of thering-shaped preliminarily molded pieces; drawing each of the ring-shapedpreliminarily molded pieces out of the transferable metal die unit; andstacking in multiple layers the ring-shaped preliminarily molded piecesdrawn from the transferable metal die unit in the axial direction of thering-shaped preliminarily molded pieces.
 15. The method of manufacturinga ring magnet as recited in claim 14, including weighing the magneticpowder for filling the cavity.
 16. The method of manufacturing a ringmagnet as recited in claim 14, including moving the upper punch to alocation above the cavity and inserting the upper punch into the cavity.17. The method of manufacturing a ring magnet as recited in claim 14,including drawing each of the ring-shaped preliminarily molded piecesfrom the transferable metal die unit while pressurized in the axialdirection.
 18. The method of manufacturing a ring magnet as recited inclaim 14, including removing the magnetic powder adhering to each of thering-shaped preliminarily molded pieces in drawing the ring-shapedpreliminarily molded piece out of the transferable metal die unit. 19.The method of manufacturing a ring magnet as recited in claim 14,including stacking the ring-shaped preliminarily molded pieces with thering-shaped preliminarily molded pieces successively turned aboutcentral axes thereof, in stacking multiple layers the ring-shapedpreliminarily molded pieces which have been drawn out of thetransferable metal die unit in the axial direction of the ring-shapedpreliminarily molded pieces.
 20. The method of manufacturing a ringmagnet as recited in claim 14, including stacking the ring-shapedpreliminarily molded pieces, with axial end faces of the ring-shapedpreliminarily molded pieces turned upside down in some layers, instacking multiple layers of the ring-shaped preliminarily molded pieceswhich have been drawn out of the transferable metal die unit in theaxial direction of the ring-shaped preliminarily molded pieces.